Electronic control circuit



Sept. 19, 1967 FIG. I

FIG.2

F. F. LADD, JR

ELECTRONIC CONTROL CIRCUIT Filed Aug. 31, 1964 170 48"- -F"' I8 ss Uni'unction 64 Troosistor Control Transistor B V I Time INVENTOR. FREDERICK E LADD, JR.

BY I} WW 7M.-.ur*)/M ATTOR N EY S United States Patent 3,343,046 ELECTRONIC CONTROL CIRCUIT Frederick F. Ladd, Jr., Newbury, Mass., assignor to Anelex Corporation, Boston, Mass., a corporation of New Hampshire Filed Aug. 31, 1964, Ser. No. 393,318 4 Claims. (Cl. 317-1485) ABSTRACT OF THE DISCLOSURE A gate turn-off switch is rendered conductive for a precisely measured time period which is substantially independent of voltage supply variations by means of a unijunction transistor having its base terminals connected between the voltage supply and the fire gate of the turn-01f switch. The emitter of the unijunction is connected to an RC- network which is connected to the junction of the turnoff switch cathode and a biasing resistor such that when the switch conducts the RC network begins charging toward the cathode potential and when the network voltage reaches a predetermined fraction of the voltage across the unijunction bases the unijunction conducts, applying a turn-off pulse to the switch fire gate and discharging the RC network.

This invention relates to a novel electronic control circuit particularly useful for controlling the application of a precisely determined pulse of current to a load.

Electronic information processing systems frequently movement between some portion of the image printingapparatus and the recording means, as in photographic,

xerographic, ferromagnetographic, electromarking stylus,

or impact printers, the amount of .energy delivered to the printing transducer, as well as the time at which the energy is applied and the power consumed, determine both the quality and the location of the image. It is the primary object of my invention to facilitate the production of current pulses for these and similar applications in response -to low-level timing pulses.

A typical high speed printer is shown in US. Patent No.

2,940,385 "to Frank Rudolph House for High Speed Printer. Such printers include constantly rotating print record sheet by striking the record sheet and an associated transfer sheet against selected characters on the roll. Since print rolls are typically operated at between 600 and 1200 rpm, the timing of the energization of the solenoids is .critical if good print quality is tobe obtained. To achieve this result, the electrical control circuit energizing the hammer solenoid must supply a precisely timed pulse of a fixed high current in response to a low level actuating pulse. Since the printer must properly print any selected character regardless of the number of characters being simultaneously printed, each electrical control circuit should be relatively insensitive to such supply voltage 'variations as normally occur when different numbers of characters are being printed. Also, because of the number of control circuits required by the printer, to minimize the space required by the printed the electronic control "circuit for each printing hammer should be as compact as possible. Specific objects of my invention are to reduce .rolls on which fonts of characters are engraved, and a bank of solenoid actuated hammers arranged in a row adjacent the print roll to print a line of characters on a the size and to improve the performance of control circuits for print hammer actuators and the like.

These and other objects of the invention are realized in a compact electronic control circuit capable of properly energizing a load even though the supply voltage may vary. This control circuit is self-timed, and only requires actuation. Briefly, the electronic control circuit of my invention comprises a novel combination of a unijunction transistor, a gate turnoff switch, and a power transistor so arranged as to take advantage of the particular characteristics of each of these devices to most efficiently carry out the functions of starting, stopping, timing and delivering a pulse of current to a load such as a print hammer solenoicl in response to a low level pulse. More specifically, the power transistor is provided with a load circuit including the load to be energized, and a control circuit governed by the gate turnoff switch. The gate turnoff switch is arranged to avalanche and turn on the power transistor in response to a pulse applied to its fire gate, and in this state current is supplied to a timing circuit. The timing circuit is connected to the control circuit of the unijunction transistor to cause it to conduct a predetermined time after the avalanche of the gate turnoff switch. The load circuit of the unijunction transistor is connected to the control circuit of the gate turnofi switch to restore the latter to the non-conducting state, cutting off the power transistor, when the unijunction transistor conducts. As will appear, the timing circuit is so arranged that the time between the initial pulse, turning on current to the load, and the interruption of load current, is relatively independent of the current supplied to the timing circuit. The resulting circuit is remarkably compact and efficient, requiring far fewer external passive components than prior circuits of comparable performance, and eliminating the need for multielement bistable circuits commonly used for storage in prior circuits.

The construction and mode of operation of the control circuit of my invention will best be understood in the light of the following description, together with the accompanying drawing, of a preferred embodiment thereof. In the drawing,

FIG. 1 is a wiring diagram of a preferred embodiment of my invention, certain elements being labeled in the drawing to facilitate an understanding of the invention, and

FIG. 2 is a representation of the voltages existing at certain points in the circuit of FIG. 1.

The illustrated circuit incorporating the preferred form 'of the invention includes the following main components:

resistors 22, 24, 26 and 50 to the negative terminal 28 of the source. The base 30 of the control transistor 10 is connected to the common junction of resistors 22 and 24, and is thereby biased by the potential at this junction into a state of non-conduction.

To actuate the circuit, appropriate actuating pulses are simultaneously applied to the. input terminals A and B of a pair of diodes 32 and 34. One of these pulses, illustrated in FIG. 2 by a voltage signal A, may be relatively long, and may for example indicate, as in a high speed input printer, that a particular character on the print roll approximately in front of the hammer is to be printed. The other pulse, signal B, may be relatively short, and may for example time the energization of the printing hammer and thus control the position of the printed character on the paper. These pulses interrupt current 'flowing through diodes 32 and 34 connected respectively between the input terminals A and B and the common junction of resistors 24 and 26. The combined action of the two pulses lowers the potential between these two resistors, in turn lowering the potential between resistors 22 and 24 and biasing the base 30 of the control transistor forwardly to cause the control transistor to conduct momentarily, or for the duration of simultaneous application of the two input pulses. Conduction of the control transistor produces a pulse of current, signal C1, flowing in its grounded emitter 36 to collector 38 path. This pulse of current flows through a resistor 40 to the fire gate 42 of the gate turnoff switch 12, actuating it. The duration of the pulse C1 should always be less than .the desired time of energization of the solenoid 16, to

prevent improper operation. The gate turnoff switch 12 is a well known and commercially available four-layer semiconducting device similar in many respects to asilicon controlled rectifier. It differs from the silicon controlled rectifier in that it has turnofi gain in the fire gate 42 to cathode 44 path. Similarly, a pulse of forward current applied in the fire gate 42 to cathode 44 path will cause the gate turn-off switch to avalanche and conduct in the anode 46 to cathode 44 path when appropriately biased. When once struck in this manner, the gate turnoff switch will stay struck and conductive in the anode to cathode path. The anode and fire gate are effectively shorted together while the gate turnoff switch is in an avalanche condition, and conductive in the anode t-o cathode path. The gate turnoff switch may thereafter be rendered nonconductive by a pulse of current in the reverse direction applied in the fire gate to cathode path. This pulse need be only one-fifth to one-tenth of the current flOWll'lg 1n the anode to cathode path.

Conduction of the control transistor 10 caused by s1- multaneous application of the input pulses to the two nput terminals will apply an actuating pulse of current, signal C1, in the fire gate 42 to cathode 44 path of the gate turnoff switch, and through resistor 48 to the source of negative potential 28. This actuates or avalanches the gate turnoff switch, rendering it strongly conductive in the anode 46 to cathode 44 path, as indicated by signal D. The anode 46 is connected through a resistor 50 to the positive terminal 20. Accordingly, while the gate turnolf switch is conductive in its anode to cathode path, current flows from the terminal 20 through the resistor 50, the gate turnoff switch, and the resistor 48 to the negative terminal 28.

The base 52 of the power transistor 14 is connected to the common junction of the resistor 50 and the anode 46. Accordingly, the flow of current while the gate turnoff switch is conductive in its anode to cathode path applies a potential to the base 52 of the power transistor, rendering it conductive in its grounded emitter 54 to collector 56 path. When the power transistor 14 is conductive in its emitter to collector path, electron current flows from a source of negative potential 58 through the series connected resistor 60 and solenoid 16 to the grounded emitter of the power transistor. Since the resistor 60 is quite small, a large surge of current flows through the solenoid 16, and the resultant strong magnetic field produced by the solenoid is well adapted to the actuation of a printing hammer for a high speed printer. Because of this large current drain on the source 58, the voltage of the source will vary somewhat. A large capacitor 62 is preferably connected to source 58 in parallel with the solenoid and power transistor series circuit to provide a reserve of current sufiicient to adequately energize solenoid 16 while the power transistor 14 is conductive. In practice, a number of print hammer solenoids may be supplied by a single capacitor of sufficient capacitance. However, the supply voltage will still normally vary somewhat, in turn varying the voltage of potential source 28. Yet control of the hammer solenoid must be relatively insensitive to this variation.

To control the duration of conduction through the 4 solenoid, a unijunction transistor 18 is provided. A unijunction transistor is another four-layer known semiconductive device, behaving in many respects like a silicon controlled rectifier with a zener diode in series with the fire gate. It has an emitter 64, a first base 66, and a second base 68. The distinguishing characteristic of a unijunction transistor is that conduction in the common base path occurs not upon application of a particular signal to the emitter, but only when the emitter to first base voltage reaches a predetermined percentage of the first base to second base voltage. Because of this voltage ratio characteristic, the unijunction transistor is used to particular advantage in the present electrical control circuit. In the quiescent state of the circuit, the common base path of the unijunction transistor 18 is non-conductive and the unijunction transistor is off even though there may be a voltage between the first and second bases, because there will be no voltage between the emitter 64 and the first base 66 while the gate turnoff switch is nonconducting. When the gate turnoff switch 12 is rendered conductive, a voltage drop will occur across the resistor 48. A capacitor 70 is connected in series with a fixed resistor 72 and an adjustable resistor 74, and this series circuit is connected in parallel with the resistor 48. Thus, the capacitor 70 will be charged by the voltage drop across the resistor 48 at a rate determined by the value of the resistors 72 and 74. The emitter 64 of the unijunction transistor is connected to the common junction of the capacitor 70 and the resistor 72. By adjustment of the variable resistor 74, the time duration of the pulse of current to .the solenoid 16 can be accurately controlled.

When the control circuit is in its quiescent state, the capacitor 70 is completely discharged, having been completely discharged by the unijunction transistor as will hereinafter appear. When the control circuit is rendered conductive and current is flowing through the anode to cathode circuit of the gate turnoff switch 12 and thus through the resistor 48, the capacitor 70 will be charged as indicated in FIG. 2 by the signal E. The charging rate of the capacitor is controlled by the adjustment of the resistor 74. As the charge on the capacitor increases, the ratio of the emitter to first base voltage relative to the second base to first base voltage of the unijunction transistor will similarly be increasing. When the voltage across the capacitor 70 reaches a predetermined percentage, normally 63%, of the voltage existing between the first and second bases of the unijunction transistor 18, the unijunction will be rendered conductive. Accordingly, while the supply potential 28 may vary somewhat depending on the number of hammer solenoids being energized, the duration of conduction of the gate turnoff switch 12 will not vary appreciably because it is controlled not by the value of the supply voltage 28 but, by a voltage ratio, and both voltages of this ratio will vary in accordance with the supply voltage. This maintains quite constant the duration of conduction of the control circuit in spite of variations in the supply voltage, as is necessary.

7 Upon conduction of the unijunction transistor 18, as indicated by the signal E, the capacitor 70 will becompletely discharged by current flow between the emitter and first base of the unijunction. Conduction of the unijunction transistor supplies a pulse of reverse current, as indicated by the signal C2, to the gate turnoff switch 12 to turn it off or render it non-conducting. This in turn removes the bias from the base 52 of 'the power transistor 14, rendering it non-conducting, opening the circuit to solenoid 16, and returning the entire circuit to a quiescent state.

In summary, upon conduction of the control transistor 10, caused by simultaneous application of appropriate pulses to the input terminals, the gate turnoff switch 12 is rendered conductive in its anode46 to cathode 44 path. This both causes power transistor 14 to conduct and energize solenoid 16, and produces a voltage drop across register 48 to cause capacitor 70 to begin to charge. When the charge on capacitor 70 reaches a fixed percentage of the voltage drop across resistor 48 and thus across the common base path of the unijunction, the unijunction is rendered conductive to supply a pulse of reverse current to the gate turnoff switch, turning it off. In this manner, the power transistor 14 is automatically rendered nonconducting, blocking the flow of current through the solenoid 16.

A diode 76 is connected between the base and emitter of the power transistor 14 to control back biasing of the transistor. Another diode 78 is connected in parallel with solenoid 16,'actually between the collector 56 of the power transistor and the source 58, to dampen the reverse current occurring during the collapse of the magnetic field of solenoid 16 when the power transistor 14 is rendered non-conductive.

This preferred circuit incorporating the invention has been constructed and tested, with satisfactory results. In this circuit, components having the following characteristics were used:

Control transistor 2N254l. Gate turnoff switch 12 Z1224. Power transistor 14 2N2612. Unijunction 18 2N-2646. Potential 20 +6 volts DC. Resistor 22 6800 ohms. Resistor 24 1000 ohms. Resistor 26 4750 ohms. Potential 28 18 volts DC. Diodes 32 and 34 1N276. Resistor 40 301 ohms. Resistor 48 100 ohms. (2

watts, 5% tolerance). Resistor 50 301 ohms. Potential 58 -36 volts. Resistor 60 -3 ohms. (20

watts). Capacitor 62 2000 microfarads, 50 volts. Resistor 72 1200 ohms. Adjustable resistor 74 1000 ohms. Diode 76 1N662. Diode 78 1N537 The voltages given are with respect to ground. All resistors were /2 watt, 1 percent tolerance unless noted otherwise.

While the preferred form of the invention has been described, it is to be understood that variations may be made in its details without departing from the spirit or scope of the invention.

I claim:

1. In combination, a power transistor having a control terminal, a load terminal, and a common terminal, a first source of current having first and second terminals, a load impedance connected in series with the terminals of said source and said load and common terminals, a gate turnoff switch having an anode, a cathode and a fire gate, a second source of current having first and second terminals at first and second potentials with respect to the potential of the first terminal of said first source, a first resistor having a first terminal connected to one terminal of said second source and a second terminal, a second resistor having a first terminal connected to the other terminal of said second source and a second terminal, the anode and cathode of said gate turnoff switch being connected in series with the second terminals of said resistors whereby the second terminal of said second resistor is at a first potential or a second potential with respect to the first terminal of said first source according as current is or is not flowing in the anode to cathode path of said gate turnoff switch, an electrical connection between the second terminal of said second resistor and the control termiml of said power transistor, the potentials of said second source being selected to cause the power transistor to conduct or be cut off according as current is or is not flowing between the anode and cathode of said gate turnoff switch, respectively, a third resistor and a capacitor connected in series across said first resistor, a unijunction transistor having an emitter, a first base and a second base, said emitter and one of said bases being connected across said capacitor, the other base being connected to the fire gate of said gate turnoff switch, and switching means operable in response to an applied pulse for applying current to the fire gate of said gate turnoff switch to cause avalanche current to flow in its anode to cathode path to turn on said power transistor and produce a voltage drop across said first resistor, said unijunction transistor being caused to conduct a predetermined time later by the chanrge accumulating on said capacitor, said unijunction transistor being poled to apply a cutoff flow of current to the fire gate of said gate turnoff switch to cut off current in the anode to cathode path.

2. A control circuit, comprising a source of current, a solenoid, a power transistor having load terminals connected in series with said solenoid and said source and a control terminal for rendering the path between said load terminals conductive when biased in a predetermined sense with respect to one of the load terminals, a gate turnoff switch having a cathode, an anode and a fire gate, said gate turnoff switch being actuable by a pulse of current of a first sense applied to its fire gate to a first state in which the anode to cathode path is conducting and being actuable by a pulse of current of a second sense opposite said first sense applied to its fire gate to a second state in which the anode to cathode path is non-conducting, circuit means comprising a first resistor and a source of current connected in series with said anode and cathode, whereby a voltage appears across said resistor when said path is conducting, means controlled by said circuit means for biasing said control terminal in said predetermined sense when said path is conducting, a second resistor and a capacitor connected in series across said first resistor, a unijunction transistor having an emitter, a first base and a second base, said emitter being connected to one side of said capacitor and said first base being connected to the other side of said capacitor, means for applying a supply voltage between the bases of said unijunction transistor whereby current flows between the bases of said unijunction transistor when the voltage across the capacitor reaches a predetermined fraction of the supply voltage, means connecting one base of said unijunction transistor to the fire gate of said gate turnoff switch to apply to the first gate a pulse of current of said second sense when said bases are conducting, and means responsive to an applied pulse for applying a pulse of current of said first sense to said fire gate to render said path conducting.

3. A control circuit for a gate turn-off switch having an anode, a cathode, and a fire gate, said anode and cathode being connected to a biasing circuit including a resistor in the portion thereof which is connected to said cathode, said control circuit comprising:

means for applying a turn-on pulse to said fire gate to switch said turn-off switch to its low impedance, conducting state;

capacitance means connected to said cathode; and

a unijunction transistor having an emitter, a first base and a second base, said emitter being connected to said capacitance means and said first and second bases being connected between said fire gate and the side of said resistor away from said cathode, whereby conductance of said turn-off switch causes said capacitance means to charge toward the potential of said cathode, rendering said unijunction transistor conductive after a predetermined time to apply a turnoff potential to said fire gate and to discharge said capacitance means.

7 8 4. The control circuit of claim 3, further comprising: 7 OTHER REFERENCES variable resistance means cooperating with said capacitance means to enable selective adjustment of the Electromcs, 5, 1960, PP- 53 a time constant thereof.

. 5 Electronics, Dec. 20, 1963, p. 35.

References Cited UNITED STATES PATENTS 3,192,466 6/1965 Sylvan et a1 323-22 3,219,910 11/1965 Klirno 32l47 10 3,267,289 8/1966 Washington et a1. 30788.5

MILTON O. HIRSHFIELD, Primary Examiner.

J. A. SILVERMAN, Assistant Examiner. 

1. IN COMBINATION, A POWER TRANSISTOR HAVING A CONTROL TERMINAL, A LOAD TERMINAL, AND A COMMON TERMINAL, A FIRST SOURCE OF CURRENT HAVING FIRST AND SECOND TERMINALS, A LOAD IMPEDANCE CONNECTED IN SERIES WITH THE TERMINALS OF SAID SOURCE AND SAID LOAD AND COMMON TERMINALS, A GATE TURNOFF SWITCH HAVING AN ANODE, A CATHODE AND A FIRE GATE, A SECOND SOURCE OF CURRENT HAVING FIRST AND SECOND TERMINALS AT FIRST AND SECOND POTENTIALS WITH RESPECT TO THE POTENTIAL OF THE FIRST TERMINAL OF SAID FIRST SOURCE, A FIRST RESISTOR HAVING A FIRST TERMINAL CONNECTED TO ONE TERMINAL OF SAID SECOND SOURCE AND A SECOND TERMINAL, A SECOND RESISTOR HAVING A FIRST TERMINAL CONNECTED TO THE OTHER TERMINAL OF SAID SECOND SOURCE AND SECOND TERMINAL, THE ANODE AND CATHODE OF SAID GATE TURNOFF SWITCH BEING CONNECTED IN SERIES WITH THE SECOND TERMINALS OF SAID RESISTORS WHEREBY THE SECOND TERMINAL OF SAID SECOND RESISTOR IS AT A FIRST POTENTIAL OR A SECOND POTENTIAL WITH RESPECT TO THE FIRST TERMINAL OF SAID FIRST SOURCE ACCORDING AS CURRENT IS OR IS NOT FLOWING IN THE ANODE TO CATHODE PATH OF SAID GATE TURNOFF SWITCH, AN ELECTRICAL CONNECTION BETWEEN THE SECOND TERMINAL OF SAID SECOND RESISTOR AND THE CONTROL TERMINAL OF SAID POWER TRANSISTOR, THE POTENTIALS OF SAID SECOND SOURCE BEING SELECTED TO CAUSE THE POWER TRANSISTOR TO CONDUCT OR BE CUT OFF ACCORDING AS CURRENT IS OR IS NOT FLOWING BETWEEN THE ANODE AND CATHODE OF SAID GATE TURNOFF SWITCH, RESPECTIVELY, A THIRD RESISTOR AND A CAPACITOR CONNECTED IN SERIES ACROSS SAID FIRST RESISTOR, A UNIJUCTION TRANSISTOR HAVING AN EMITTER, A FIRST BASE AND A SECOND BASE, SAID EMITTER AND ONE OF SAID BASES BEING CONNECTED ACROSS SAID CAPACITOR, THE OTHER BASE BEING CONNECTED TO THE FIRE GATE OF SAD GATE TURNOFF SWITCH, AND SWITCHING MEANS OPERABLE IN RESPONSE TO AN APPLIED PULSE FOR APPLYING CURRENT TO THE FIRE GATE OF SAID GATE TURNOFF SWITCH TO CAUSE AVALANCHE CURRENT TO FLOW IN ITS ANODE TO CATHODE PATH TO TURN ON SAID POWER TRANSISTOR AND PRODUCE A VOLTAGE DROP ACROSS SAID FIRST RESISTOR, SAID UNIJUNCTION TRANSISTOR BEING CAUSED TO CONDUCT A PREDETERMINED TIME LATER BY THE CHARGE ACCUMULATING ON SAID CAPACITOR, SAID UNIJUNCTION TRANSISTOR BEING POLED TO APPLY A CUTOFF FLOW OF CURRENT TO THE FIRE GATE OF SAID GATE TURNOFF SWITCH TO CUT OFF CURRENT IN THE ANODE TO CATHODE PATH. 